2-1. Heat Forming Process

• 2-1-1. Heating Forming Temperature

  The normal temperature for forming acrylic sheet is about 160°C (320°F), however if the part being formed has small sharp radii, it may be necessary to raise the temperature step by step to a maximum of 200°C (392°F) providing the sheet has not been exposed to moisture or high humidity.

• 2-1-2. Precautions of each Heating Method

  Regardless of the type of oven used, forced air or electric infrared, it is important to raise the sheet temperature step by step. Only when the entire sheet is heated thoroughly that is, the top, middle and bottom to a sufficient forming temperature is it permissible to form the sheet.

  1. Forced Air Circulating Oven

     1. Forced Air Circulating Oven Since a fan evenly distributed the heated air, throughout the oven you can get good uniform forming temperatures.

        
        

        Fig. 2-1. Heating Speed of SHINKOLITE(R)-A PX200

        

     2. Electric Infrared Ray Oven

        In the use of this oven for mass production, some problems may occur. The following are some of these problems and possible preventative measures to take.

        1. The difference in temperature, particularly in a thicker sheet, between the surface of a sheet and the core during the heating or warming up of the sheet. This problem can cause overheating on the surface or misforming because the core is too cold. The solution to this problem being to lower the voltage and heat the sheet more slowly.

           
           

           Fig. 2-2. Difference of arising temperature speed between a surface and a centre of SHINKOLITE-A caused by Electric Infrared ray heating

           

        2. The outside circumference for the sheet will not get enough heat. This may be caused by the clamping device cutting the infrared ray and transferring the heat away from the sheet into the metal causing the sheet to stretch unevenly resulting in thin bottom radii. The solution to this problem may be two-fold.

           1. Masking

              By suspending some wire net between the heating elements and the center portion of the sheet, you can more evenly heat the sheet.

           2. Having a zone control capability in the oven, you can lower the heating temperatures and times in the center portion of the sheet.
• 2-1-3. Other Cautions

  Please keep in mind these special properties of acrylic resin when you heat it.

  1. Sheet shrinkage due to heating

     Cast acrylic sheet shrinks about 2% after the first heating. When you cut a sheet, it is necessary to consider the sheet shrinkage and to have some allowance depending upon your choice of forming method and shape of formed products. With a forced air circulating oven special care should be taken in sheet size selection. Due to the fact that you must heat the sheet without clamping.

  2. Bubbles or blisters due to over-heating

     Different colors react to heat differently. Some colors are more easily overheated to the point of bubbles or blisters. In addition leaving acrylic sheet uncovered and open to moisture will cause the bubbles to appear at a lower temperature. Acrylic sheet is hydroscopic and can absorb moisture. Please keep stored in a dry warehouse leaving the polyethylene film attached until just prior to insertion into the oven.

2-2. Fiberglassing Process

• 2-2-1. Preparation

  After the "part" is formed you should apply some masking tape or protective covering to the top of the formed parts. This will protect from abrasions in transportation as well as overspray in the next step.

• 2-2-2. Placement of the formed part on the jig.

  This male jig is made of plastic or wood and used to support the part in an upside down position as the part travels through the spray-up procedure. Careful attention should be given to support the thin corners or radii of the part to keep them from cracking during the roll out phase of the glassing procedure.

  
  

  Fig. 2-3, 2-4.

  

  In each way well-balanced supporters should be applied.

• 2-2-3. Resin Selection and Preparation

  Of the several types of resin used in the fiberglass industry the isopthalic type is preferred for its bonding characteristics over the orthopthalic type. The quantity of catalyst (MEKPO) should be carefully adjusted considering such factors as outside air temperature and humidity as well as needed work time on the part.

  1. When a resin is over catalyzed especially in a high outside temperature situation, it is likely that poor adhesion would result as well as causing a high exotherm in the laminate ultimately perhaps warping or bubbling.
  2. When a resin is under catalyzed especially in a low outside air temperature situation, the laminate may gel or polymerize too slowly resulting in a styrene attack on the sheet causing cracks. (Refer to 3-2-7)
• 2-2-4. Selection of Fiberglass

  The cloth or rovings used will normally get better adhesion if the sizing (binder) on the glass is pure silane.

• 2-2-5. Preparation of the Acrylic Surface.

  In many cases the surface of the acrylic sheet to be glassed is wiped with IPA (isopropyl Alcohol). In the thicker sheets a sand paper wipe to scratch the surface results in better adhesion.

• 2-2-6. Fiberglassing Procedures.

  In the spray-up method of fiberglassing it is advisable to spray a thin skin coat onto the acrylic surface. This has two advantages.

• 2-2-7. Problems and Solutions in Fiberglassing

  Problem	Cause	Solution
  Cracking and swelling	Shortage of heating (stress in a formed part) Air bubbles between acrylic sheet and polyester resin High temperature in polymerization of polyester resin	[forming condition] To check the forming temperature and other conditions To check the thickness of thin parts and keep it thicker than 0.8mm (0.031") [Polyester resin] To avoid thick fiberglass in one time To put out air bubbles fully with a roller To use a suitable polyester resin for each season To adjust a catalyst
  Bad mixing of polyester resin and fiberglass	Shortage of polyester resin Over viscosity of polyester resin Disqualified glass roving	Standard quality of fiberglass is at 20-35% against polyester resin To use suitable viscous polyester resin To select treated fiberglass with silane for polyester resin
  Slipping down of polyester resin or fiberglass with polyester resin	Shortage of fiberglass Too short chopped fiberglass Too short distance between spray gun and formed part	Standard quantity of fiberglass is at 20-35% against polyester resin. To use longer chopped fiberglass To make a sufficient distance between spray gun and formed part.
  Insufficiency of polymerization	Shortage of catalyst Too low atmospheric temperature	To increase catalyst To raise the room temperature